Introduction: Common indications for temporo mandibular joint prothesis are ankylosis, tumours or dysplasia. Therapeutic options are the surgical insertions of osteochondral autologous rib transplants, alloplastic titan prosthesis or total endo prostesis in combination with polyethylene as chondral replacement. Patient and therapy: We report on a patient with bilateral congenital aplasia of the condylar processus. At the age of 10 years she was provided with alloplastic titan prosthesis with spheric joints by a combined intra- and extraoral approach. The functional and aesthetic results were good. After 18 years of use the routinely performed CT scan showed an intracranial dislocation of the titan prosthesis through the glenoid fossa into the middle cranial fossa with close anatomical relation to the temporal flap. Results: The patient presented no neurologic symptoms; the function of the temporo mandibular joint was unaffected. The SKD was 35 mm. The patient had no pain or joint sounds. There was no lateral deviation during the opening movement. Due to the unaffected joint function and the lack of restraining symptoms, the patient wishes no surgical intervention to remove the dislocated prosthesis and replace them by a total endoprosthesis. Conclusion: Autologous and alloplastic joint replacement represent a reliable therapeutic intervention in the functional and aesthetic rehabilitation of temporo mandibular joint ankylosis, dysplasia or degenerative joint disease. Continuous and pressures may lead to a bony degradation of the glenoid fossa and the dislocation of the prosthesis. Alloplastic transplants should be replaced after two years by an autologous transplant or a total prosthesis, according to their accreditation. Schlagwörter: Kiefergelenk, Dislokation, Perforation, Gelenkersatz